Shape of the root canal after ultrasonic instrumentation with K-Flex files

Ahmad M. Shape of the root canal after ultrasonic instrumentation with K-Flex files. Endod Dent Traumatol 1990; 6: 104-108. Abstract - This study investigated the feasibility of using K-Flex files with the Cavi-Endo ultrasonic unit. The effectiveness of the K-Flex files in shaping canals was compared with the ultrasonic Cavi-Endo K-files. Simulated curved canals in clear resin blocks were instrumented ultrasonically using the 2 file types. Pre- and postinstrumentation photographs were taken and the following were evaluated: apical and coronal areas, the incidence of the narrowing of the canals coronal to the apical transport (elbows). Displacement amplitude measurements were carried out in order to see any relationship between the incidence of elbows and displacement amplitudes of the files. The results indicated that the K-Flex files were more efficient than the ultrasonic K-files and resulted in less elbow formation. No direct relationship was observed between displacement amplitudes and the incidence of elbow formation. It is suggested that the manufacturers of ultrasonic units consider different file designs. iViajinaii Alimad
Department of Conservative Dentistry, University of Malaya, Kuala Lumpur, Malaysia

The introduction of ultrasonic instrumentation and its acclaimed versatility was innovative, timely and generated tremendous interest. When it was first introduced, it was hoped some shortcomings of hand instrumentation would be overcome. Although the conventional hand instrumentation has been the mainstay in endodontics for a number of years, it involves a procedure that is technically demanding and time-consuming. Various studies have been carried out comparing the effectiveness of ultrasonic instruments with that of hand instrumentation in shaping canals. The results have been conflicting; some authors report that ultrasonic instrumentation was more efficient in shaping curved canals than hand instruments (1), while others found the contrary (2). Tronstad & Niemczyck (3) compared the effectiveness of various types of automated instruments and found that the ultrasonic instrument system failed to perform well. To improve the performance of the instrument, they suggest that the manufacturer consider a change in file design. This view was supported in a recent study (4) that reported that ultrasonic K-files are similar to hand K-files in terms of cutting efficiency. A recent addition to the endodontic armamentarium has been the K-Flex file. It has been stated that the K-Flex file is more efficient in cutting than the K-file (5). The present 104

study was undertaken to evaluate the possibility of using the K-Flex file as an ultrasonic file. Material and metiiods Simulated curved canals in clear resin blocks made from clear casting resin (L.R. White, London Resin, Basingstoke, Hampshire, UK) were employed. Silver points bent to a 30° curvature were used as moulds for the canals. Instrumentation of canals and photographic technique employed Fifty canals were divided into 10 groups of 5 canals each and subjected to different times of instrumentation using either the ultrasonic K-files supplied with the Cavi-Endo unit (Caulk, Milford, DE, USA) or the K-Flex files (Kerr Sybron, Romulus, MI, USA). The K-Flex files used were hand files 25 mm in length whose handles were removed using a carborundum disc. All the files were precurved prior to instrumentation. A photograph of each canal was taken before instrumentation (Fig. 1). This was done by placing the block on the stage of a photographic enlarger. The position of the enlarger head was adjusted until

For each canal investigated.1. It was considered that if the canals exhibiting this phenomenon were included in the assessment of the areas. In addition.and postinstrumented prints. all postinstrumented prints were considered. Filing was done in a light push-pull stroke along the walls followed by circumferential movement with continuous flow of water. The time of instrumentation at which the elbow occurred was recorded. A line drawn across the image of the canal 5 mm from the apex was taken as the dividing line between the apical and the coronal halves of the canal. The ultrasonic unit used was the Cavi-Endo. Filing was commenced with a size 15 file to the full working length of the canal. In groups 1 to 5. 3 representative samples. the displacement amplitudes of the files were quantified. Measurements of the apical and coronal areas were made from the pre. Instrumentation was then continued using size 20 file and a further photograph was made. were investigated. Coronal areas removed following instrumentation with size 25 (K-Flex vs K-file). Files of size 15. a photograph was taken. The prints were assessed for the incidence of elbow formation. For the coronal areas. Photograph of simulated canal prior to instrumentation. Measurements were repeated 5 times for each file and the mean calculated. the canals were instrumented iiltrasonically with the K-file. A power setting of 1 was used during the instrumentation. The displacement amplitude values for file sizes 15.
The pattern of removal of dentine did not differ between the 2 file types (Fig. When the file was set into oscillation. the light was seen as a thin transverse line. the size of the file used that resulted in the elbow formation was noted.
Qualitative and quantitative assessment of prints
The images of the canals before and after instrumentation were traced onto cellulose acetate sheets and subjectively evaluated for removal of material from the canal walls. a total of 4 photographs were taken. The incidence of elbows following instrumentation with size 25 for both groups. 2). The file was positioned under the eyepiece of a travelling microscope and illuminated from the side until a pinpoint source of light was seen at the very tip. Ultrasonic filing was carried out according to the technique recommended by the manufacturer. The following groups were subjected to statistical analysis using Student's t-test: 1 Apical areas removed following instrumentation with size 15 (K-Flex vs K-file) Apical areas removed following instrumentation with size 20 (K-Flex vs K-file) Apical areas removed following instrumentation with size 25 (K-Flex vs K-file) 4. Coronal areas removed following instrumentation with size 20 (K-Flex vs K-file) 6. 20 and 25 were used for 1 min each in group 1.Root canal shape following ultrasonic instrumentation
the X 7 image of the canals was projected onto a sheet of photographic paper. 9 min for group 3 and 12 and 15 min for groups 4 and 5 respectively. this was the constriction of the canal. Coronal areas removed following instrumentation with size 15 (K-Flex vs K-file) 5. relatively narrower than those parts on either side of it and coronal to an apical transportation.and K-Flex files was statistically evaluated using the chi-square test. 3 min in group 3. the results would not reflect the true cutting ability of the instruments. The canal was subsequently instrumented with a size 25 file and then a final photograph was taken. The experimental arrangement for the measurement of the displacement amplitude has been described elsewhere (6). For a particular file size. each selected from a different batch number.
Measurement of the displacement amplitudes
In order to see any relationship between displacement amplitude and the incidence of elbow formation. Following this. 20 and 25 were quantified. This gave a total instrumentation time of 3 min for group 1. half of this line gave the value of the displacement amplitude.
Results Qualitative assessment
. Measurements of areas and distances were made
using a digitizer tablet and a computer. 6 min for group 2. K. This same sequence was carried out with the K-Flex files in groups 6-10. All canals exhibited
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. 4 min in group 4 and 5 min in group 5. 2 min in group 2. Measurements of apical areas was limited to the canals that did not exhibit lateral movement of the foramen (apical transportation).

2. more removal occurred at the inner curve. Flaring of the canal instrumented with the K-Flex file is more pronounced. while some removal was seen on the outer wall. filing at shorter times of instrumentation using size 15 Kfiles failed to enlarge the canal sufficiently to allow the size 20 files to be introduced to the full working length. 3). the opposite occurred. In general. The more flexible size 15 files did not exhibit
Eig. No such observation was encountered when K-Flex files were used. 3. it became evident that the K-Flex files gave a more pronounced coronal flaring than the K-file. the K-Flex files were slightly more efficient than the K-file (NS).
unequal removal along the canal walls with most of the removal occurring at the coronal ends. Quantitative assessment
3 Minutes 6 Minutes 9 Minutes 12 Minutes 15 Minutes
Fig.
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. Composite tracings of canals subjected to ultrasonic instrumentation using K-files and K-Flex files.2). apically. the pattern of removal changed. In the coronal areas of the canals. 2 or 3 min failed to enlarge the canal sufficiently to allow the size 20 file to be negotiated easily to the full working length. elbows formed only after 15 min of instrumentation. while the outer portion of the curve experienced little or no removal. Elbow formation was affected by the size of the file and size 15 files did not cause elbow formation in any group. Secondly.
Quantitative assessment supported the observation that more material was removed coronally than apically in the root canals (Tables 1. The K-file gave a higher incidence of elbows than the K-Flex file (P<0. the K-Flex files removed more material from the walls of the canals than the K-files. Typical examples of canals instrumented with (A) K file and (B) K-Flex file. It also caused more apical transportation.01). First. No such observation was encountered with the K-Flex files.01). incidence of elbow formation The incidence of elbow formation in the various groups is shown in Table 3. and little removal was observed apically. Dispiacement ampiitude values The values of the displacement amplitudes and the incidence of elbows are shown in Table 4. In addition. Discussion Although the pattern of removal of material from canal walls was similar with both file types. Most elbow formations were seen following instrumentation with size 25 files. the K-Flex file performed better than the K-file (i^<0. With the ultrasonic K-Flex group. the K-Flex files were observed to result in significantly less elbow formation than the K-files. In the apical region. Intergroup comparisons showed that. an increase in instrumentation time increased coronal flaring and increased removal from the wall of the inner curve of the canal. A relationship existed between the time of instrumentation and the incidence of elbow formation. there appeared to be equal removal of material from both the mesial and distal sides of the walls. Apically. As the canal curved at the mid-region of the root. little or no removal was evident at the inner wall. At the coronal ends. With an increase in instrumentation time. the amount of material removed did not differ in the groups. Initial instrumentation with size 15 K-files at shorter times of instrumentation of either 1. several findings from this study highlighted the better cutting efficiency of the K-Flex files.Ahmad
Canals instrumented with K-files were narrower than those prepared using K-Flex files (Fig. No direct relationship existed between the displacement amplitude and the incidence of elbow formation.